Background: Spinal fractures related to AS are often treated by long posterior stabilisation. The biomechanical rationale behind is the neutralisation of long lever arms in the ankylosed spine to avoid non-union or neurological deterioration. Despite the widespread application of long posterior instrumentation it has never been investigated in a biomechanical model. The objective of this study is to develop a finite element model for spinal fractures related to AS and to establish a biomechanical foundation for long posterior stabilisation of cervicothoracic fractures related to ankylosing spondylitis (AS).
Methods: An existing finite element-model (consisting of two separately developed models) including the cervical and thoracic spine were adapted to the conditions of AS (all discs fused, C0-C1 and C1-C2 mobile) and a fracture at the level C6-C7 was simulated. Besides a normal spine (no AS, no fracture) and the uninstrumented fractured spine four different posterior transpedicular instrumentations were tested: 1. Fracture uninstrumented, 2. Short instrumentation C6-C7, 3. Medium instrumentation C5-T1, 4. Long instrumentation C3-T3, 5. Skipped level long instrumentation C3-C6-C7-T3.
Three loads (1.5g, 3.0g, 4.5g) were applied according to a specific load curve. Kinematic data such as the gap distance in the fracture site were obtained. Furthermore the stresses in the ossified parts of the discs were evaluated.
Findings: All posterior stabilisation methods could normalise the axial stability at the fracture site as measured with gap distance. With larger accelerations than 1.5g , it was seen that the longer instrumentations resulted in lesser maximal gap distance than the Short instrumentation. The maximum stress at the cranial instrumentation end (C3-C4) was slightly greater if every level was instrumented, than in the skipped level model. The skipped level instrumentation achieved similar rotatory stability as the long multilevel instrumentation.
Interpretation: The FE model developed simulated a spinal fracture at C6-C7 level. Skipping instrumentation levels without giving up instrumentation length also reduces the stresses in the ossified tissue within the range of the instrumentation and does not decrease the stability in a finite element model of a cervicothoracic fracture related to AS. Considering the risks associated with every additional screw placed, the skipped level instrumentation has advantages with regard to patient safety. The effects of the degree of osteoporosis, screw placement and pre-existing kyphosis on the construct stability were not investigated in this study and should be a matter of further research.